You are invited to come to see the Earth turn, tomorrow, from three to five, at Meridian Hall of the Paris Observatory.

Invitation cards which he sent out to the scientists of Paris, to witness his famous pendulum experiment on 3 February 1851, as quoted in Pendulum : Léon Foucault and the Triumph of Science (2003) by Amir D. Aczel

The phenomenon develops calmly, but it is invisible, unstoppable. One feels, one sees it born and grow steadily; and it is not in one's power to either hasten it or slow it down. Any person, brought into the presence of this fact, stops for a few moments and remains pensive and silent; and then generally leaves, carrying with him forever a sharper, keener sense of our incessant motion through space.

On his pendulum experiment, as quoted in Pendulum : Léon Foucault and the Triumph of Science (2003) by Amir D. Aczel

To contribute usefully to the advance of science, one must sometimes not disdain from undertaking simple verifications.

As quoted in The Life and Science of Léon Foucault : The Man Who Proved the Earth Rotates (2003) by William Tobin, p. 72, ISBN 0521808553

May those men who are animated by the true scientific spirit cease to believe that they are obliged to consider only subjects of public utility; ...because, one must not hide it, the field of applications, so rich and fertile today, would not take long to be blighted by sterility if it ceased to be fertilized and revivified by the beneficial light which theoretical research radiates and pours incessantly upon it.

As quoted in The Life and Science of Léon Foucault : The Man Who Proved the Earth Rotates (2003) by William Tobin, p. 93.

At the age of 25, not having learnt anything at school nor from book, enthusiastic about science but not about study, Léon Foucault took on the task of making the work of scientists understandable to the public and of passing judgement on the value to the work of leading men of science. From the start he showed great subtlety, good judgement based on more prudence than would be expected. His first articles were remarkable; they were spiritual. He took his duties seriously. Launched, without any experience, into the highest level of science with all its confusion and problems, he was assured carrying out a role in which mediocrity would mean failure, with complete success.
… Always polite, yet seeking the truth, Foucault applied carefully considered judgements. Previously an unknown, this young man with no scientific publications nor known scientific discoveries, displayed a quiet authority and frankness which irritated many leading scientists.